Electrical transport properties and Kondo effect in La_1-xPr_xNiO_3-δ thin films

The Kondo effect has been a topic of intense study because of its significant contribution to the development of theories and understanding of strongly correlated electron systems. In this work, we show that the Kondo effect is at work in La_1-xPr_xNiO_3-δ (0 ≤ x ≤ 0.6) thin films. At low temperatures, the local magnetic moments of the 3d e_g electrons in Ni²⁺, which form because of oxygen vacancies, interact strongly with itinerant electrons, giving rise to an upturn in resistivity with x ≥ 0.2. Observation of negative magnetoresistance, described by the Khosla and Fisher model, further supports the Kondo picture. This case represents a rare example of the Kondo effect, where Ni²⁺ acts as an impurity in the background of Ni³⁺. We suggest that when Ni²⁺ does not participate in the regular lattice, it provides the local magnetic moments needed to scatter the conduction electrons in the Kondo effect. These results offer insights into emergent transport behaviors in metallic nickelates with mixed Ni³⁺ and Ni²⁺ ions, as well as structural disorder.